Dc-dc converter and manufacturing method thereof

ABSTRACT

A DC-DC converter is driven by single high input voltage, and includes a voltage converter circuit and a control circuit. The increase of the occupied area of the DC-DC converter is suppressed. The DC-DC converter includes an input terminal to which input voltage is applied; a voltage converter circuit connected to the input terminal, and including a first transistor; a control circuit configured to control the voltage converter circuit, and including a second transistor including a silicon material in a channel formation region; and a third transistor provided between the input terminal and the control circuit, and configured to convert the input voltage into power supply voltage that is lower than the input voltage. The first transistor and the third transistor include an oxide semiconductor material in channel formation regions. The first transistor and the third transistor are stacked over the second transistor with an insulating film provided therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the disclosed invention relates to a DC-DC converterand a manufacturing method thereof.

2. Description of the Related Art

In recent years, DC-DC converters have been widely used in order todrive circuits which are driven by DC voltage that is different frominput voltage because devices which need DC voltage sources have beendiversified (see Patent Document 1, Patent Document 2, and PatentDocument 3).

[Reference]

-   [Patent Document 1] Japanese Published Patent Application No.    2009-148129-   [Patent Document 2] Japanese Published Patent Application No.    2003-235251-   [Patent Document 3] Japanese Published Patent Application No.    2009-254110

SUMMARY OF THE INVENTION

A DC-DC converter includes a voltage converter circuit and a controlcircuit which controls the voltage converter circuit. A power devicewhich has high breakdown voltage is used for an element of the voltageconverter circuit because high voltage can be applied. Such a voltageconverter circuit using a power device can be driven only by high inputvoltage. As such a power device, an element including an oxidesemiconductor material (e.g., a transistor which uses an oxidesemiconductor film in a channel formation region) can be given forexample.

In contrast, an element of the control circuit which controls thevoltage converter circuit needs a high driving speed. As such an elementwhich can provide a high driving speed, an element using a siliconmaterial (e.g., a transistor formed using a silicon film for a channelformation region or a transistor formed using a single crystal siliconsubstrate) can be given as an example. However, high voltage cannot beapplied to the element using a silicon material because the element doesnot have high breakdown voltage.

Since silicon has a narrow band gap, an avalanche breakdown in whichelectrons are generated like an avalanche might occur. When an avalanchebreakdown occurs, the element is broken. In contrast, the oxidesemiconductor has a wide band gap; thus, the avalanche breakdown doesnot easily occur and the element is less likely to be broken.

Here, the case where a DC-DC converter having a voltage convertercircuit and a control circuit is driven only by single high inputvoltage is considered. The single input voltage is preferable in orderto reduce an occupied area of the DC-DC converter. However, although thehigh input voltage can drive the voltage converter circuit, the highinput voltage might break the control circuit because it is too high asthe voltage of the control circuit. Accordingly, it is difficult todrive the DC-DC converter having a voltage converter circuit and acontrol circuit only by the single high input voltage.

Therefore, an element which decreases high input voltage is providedbetween an input terminal to which the high input voltage is applied andthe control circuit. When the decreased voltage is supplied to thecontrol circuit, there is no risk of breaking the control circuit.

The above-described power device may be used as such an element whichdecreases the high input voltage. However, the provision of the controlcircuit and the power device which decreases the voltage value of thehigh input voltage might increase the occupied area of the DC-DCconverter.

In view of the above problems, an object of one embodiment of thedisclosed invention is to provide a DC-DC converter which is driven bysingle high input voltage and includes a voltage converter circuit and acontrol circuit.

In addition, an object of one embodiment of the disclosed invention isto inhibit the increase of the occupied area of the DC-DC converter.

Further, an object of one embodiment of the disclosed invention is todecrease the number of manufacturing steps and manufacture cost in sucha way that a power device of a voltage converter circuit and a powerdevice which decreases input voltage are formed in the same process.

As the power device which decreases the voltage value of the single highinput voltage and the power device of the voltage converter circuit,elements including an oxide semiconductor material which aresemiconductor elements with high breakdown voltage are used; and as anelement of the control circuit, a semiconductor element with lowbreakdown voltage, for example, an element using a silicon material isused. In addition, the power devices and the element of the controlcircuit overlap with each other.

Even when the semiconductor element with low breakdown voltage, forexample, an element using a silicon material is used as the element ofthe control circuit, the power device decreases the voltage value of thehigh input voltage; therefore, there is no risk of damaging the controlcircuit.

It is possible to inhibit the increase of the occupied area of the DC-DCconverter in order that the power devices and the element of the controlcircuit overlap with each other.

In addition, in one embodiment of the disclosed invention, the elementincluding an oxide semiconductor material which is the power device ofthe voltage converter circuit and the element including an oxidesemiconductor material which is a power device that decreases the inputvoltage are formed in the same process. Thus, the number ofmanufacturing steps and the manufacture cost can be reduced.

One embodiment of the disclosed invention relates to a DC-DC converterincluding: an input terminal to which input voltage is applied; avoltage converter circuit that is connected to the input terminal andincludes a first transistor; a control circuit that is configured tocontrol the voltage converter circuit and includes a second transistorincluding a silicon material in a channel formation region; and a thirdtransistor that is provided between the input terminal and the controlcircuit and configured to convert the input voltage into power supplyvoltage that is lower than the input voltage. The first transistor andthe third transistor are transistors including an oxide semiconductormaterial in channel formation regions, and the first transistor and thethird transistor are stacked over the second transistor with aninsulating film provided between the first and third transistors and thesecond transistor.

One embodiment of the disclosed invention relates to a method formanufacturing a DC-DC converter, including the steps of: forming a firsttransistor using a silicon material in a first channel formation region,over an insulating surface; forming an insulating film covering thefirst transistor; forming a second transistor including an oxidesemiconductor material in a second channel formation region and a thirdtransistor using the oxide semiconductor material in a third channelformation region over the insulating film; in which a voltage convertercircuit includes the second transistor; a control circuit configured tocontrol the voltage converter circuit includes the first transistor; andthe third transistor is provided between an input terminal and thecontrol circuit, and converts input voltage applied to the inputterminal into power supply voltage that is lower than the input voltage.

According to one embodiment of the disclosed invention, the oxidesemiconductor material is any one of an In—Sn—Ga—Zn—O-based oxidesemiconductor which is a four-component metal oxide; an In—Ga—Zn—O-basedoxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, aSn—Al—Zn—O-based oxide semiconductor which are three-component metaloxides; an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, an In—Ga—O-based oxide semiconductorwhich are two-component metal oxides; an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductorwhich are one-component metal oxides.

According to one embodiment of the disclosed invention, the voltageconverter circuit is a step-down voltage converter circuit.

According to one embodiment of the disclosed invention, the voltageconverter circuit is a flyback voltage converter circuit.

According to one embodiment of the disclosed invention, the voltageconverter circuit is a forward voltage converter circuit.

Note that the ordinal numbers such as “first” and “second” are used forconvenience and do not denote the order of steps and the stacking orderof layers. In addition, the ordinal numbers in this specification do notdenote particular names which specify the present invention.

According to the disclosed invention, a DC-DC converter which is drivenby single high input voltage and has a voltage converter circuit and acontrol circuit can be obtained.

According to one embodiment of the disclosed invention, the increase ofthe occupied area of the DC-DC converter can be suppressed.

In one embodiment of the disclosed invention, the number ofmanufacturing steps and manufacture cost can be decreased because apower device of a voltage converter circuit and a power device whichdecreases input voltage are formed in the same process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a DC-DC converter.

FIG. 2 is a circuit diagram of a DC-DC converter.

FIG. 3 is a circuit diagram of a DC-DC converter.

FIG. 4 is a circuit diagram of a DC-DC converter.

FIGS. 5A to 5C are cross-sectional views illustrating a layeredstructure of a transistor.

FIGS. 6A to 6C are cross-sectional views each illustrating a layeredstructure of a transistor.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed in this specification will behereinafter described with reference to the accompanying drawings. Notethat the invention disclosed in this specification can be carried out ina variety of different modes, and it is easily understood by thoseskilled in the art that the modes and details of the invention disclosedin this specification can be changed in various ways without departingfrom the spirit and scope thereof. Therefore, the present invention isnot construed as being limited to description of the embodiments. Notethat, in the drawings hereinafter illustrated, the same portions orportions having similar functions are denoted by the same referencenumerals, and repeated description thereof will be omitted.

Note that in the invention disclosed in this specification, asemiconductor device refers to an element or a device which functions byutilizing a semiconductor and includes, in its category, an electricdevice including an electronic circuit, a display device, alight-emitting device, and the like and an electronic appliance on whichthe electric device is mounted.

<Circuit Structure>

A DC-DC converter illustrated in FIG. 1 includes a control circuit 111,a voltage converter circuit 121, an input terminal 102 to which inputvoltage Vin is applied, a transistor including an oxide semiconductormaterial in a channel formation region, for example, a transistor 101which is a transistor including an oxide semiconductor film in a channelformation region (hereinafter referred to as an oxide semiconductortransistor), and an output terminal 131 which outputs output voltageVout which is output from the voltage converter circuit 121.

One of a source and a drain of the transistor 101 is connected to theinput terminal 102 to which the input voltage Vin is applied and one ofa source and a drain of a transistor 124 in the voltage convertercircuit 121. The other of the source and the drain and a gate of thetransistor 101 are connected to the control circuit 111.

The transistor 101 which is an oxide semiconductor transistor is a powerdevice which can withstand high voltage. The input voltage Vin appliedto the one of the source and the drain of the transistor 101 isconverted into power supply voltage VDD having a value which is lowerthan a value of the input voltage Vin based on gate voltage applied tothe gate of the transistor 101.

As the input voltage Vin, voltage from a power source for family usewhich is subjected to AC-DC conversion is used for example. When thevoltage from a power source for family use which is subjected to AC-DCconversion is used as the input voltage Vin, an effective value of theinput voltage Vin is 141 V. Therefore, the input voltage Vin which issuch high voltage might damage the control circuit 111. Therefore, theprovision of an oxide semiconductor transistor which is a power devicewith high breakdown voltage between the input terminal 102 to which theinput voltage Vin is applied and the control circuit 111 can prevent thedamage of the control circuit 111.

The transistor 101 is a transistor including an oxide semiconductor filmin a channel formation region as described above.

As a material used for such an oxide semiconductor film, the followingoxide semiconductors can be used: an In—Sn—Ga—Zn—O-based oxidesemiconductor which is a four-component metal oxide; an In—Ga—Zn—O-basedoxide semiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, aSn—Al—Zn—O-based oxide semiconductor which are three-component metaloxides; an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, an In—Ga—O-based oxide semiconductorwhich are two-component metal oxides; an In—O-based oxide semiconductor,a Sn—O-based oxide semiconductor, and a Zn—O-based oxide semiconductorwhich are one-component metal oxides; or the like. Note that a structureand a manufacturing process of a transistor (an oxide semiconductortransistor) having an oxide semiconductor film in a channel formationregion will be described later.

The control circuit 111 is electrically connected to the other of thesource and the drain of the transistor 101, the gate of the transistor101, and the voltage converter circuit 121. Further, power supplyvoltage VSS having a lower value than power supply voltage VDD isapplied to the control circuit 111. As the power supply voltage VSS,ground voltage GND may be used for example.

FIG. 2 is an example of a detailed circuit configuration of the controlcircuit 111 illustrated in FIG. 1.

The control circuit 111 in FIG. 2 includes an internal voltagegeneration circuit (also referred to as a “regulator”) 110 and aninternal control circuit 130. The internal voltage generation circuit110 is a circuit which generates the power supply voltage VDD andincludes an operational amplifier 112, a resistor 113, and a resistor114.

An inverting input terminal of the operational amplifier 112 iselectrically connected to one terminal of the resistor 113 and oneterminal of the resistor 114. A non-inverting input terminal of theoperational amplifier 112 is electrically connected to a terminal 115 towhich reference voltage Vref is applied. An output terminal of theoperational amplifier 112 is connected to the gate of the transistor101.

Transistors included in the operational amplifier 112 may be transistorswhich can be driven at high speed, for example, transistors including asilicon material in a channel formation region. Note that a structureand a manufacturing process of the transistors in which a siliconmaterial is used for a channel formation region will be described later.

In the case where a silicon material is used for a channel formationregion, an n-channel transistor and a p-channel transistor can beobtained. When transistors including silicon in a channel formationregion are used as the transistors included in the operational amplifier112, the transistors can be driven at high speed, and can be ann-channel transistor or a p-channel transistor.

The one terminal of the resistor 113 is electrically connected to theinverting input terminal of the operational amplifier 112 and the oneterminal of the resistor 114. The other terminal of the resistor 113 iselectrically connected to the other of the source and the drain of thetransistor 101 and a first terminal of the internal control circuit 130.

The one terminal of the resistor 114 is electrically connected to theone terminal of the resistor 113 and the inverting input terminal of theoperational amplifier 112. The power supply voltage VSS is applied tothe other terminal of the resistor 114.

The internal control circuit 130 is a circuit which performs voltagecontrol or current control. As an example of the voltage control or thecurrent control, a pulse width modulation (PWM) control and a hysteresiscontrol can be given, for example. In this embodiment, the power supplyvoltage VDD is converted into gate voltage applied to a gate of thetransistor 124, by the internal control circuit 130. In accordance withthe gate voltage, the input voltage Vin is converted into the outputvoltage Vout.

The first terminal of the internal control circuit 130 is electricallyconnected to the other of the source and the drain of the transistor 101and the other terminal of the resistor 113. The power supply voltage VSSis applied to a second terminal of the internal control circuit 130. Inaddition, a third terminal of the internal control circuit 130 iselectrically connected to the gate of the transistor 124. Although notillustrated, part of the output voltage Vout is fed back to the internalcontrol circuit 130. Such part of the output voltage Vout may begenerated in such a way that a voltage divider which is electricallyconnected to the output terminal 131 which outputs the output voltageVout is provided and the output voltage Vout is divided by the voltagedivider.

The voltage converter circuit 121 in FIG. 2 is a step-down voltageconverter circuit. The voltage converter circuit 121 in FIG. 2 includesthe transistor 124, a diode 123, a coil 122, and a capacitor 125.

The one of the source and the drain of the transistor 124 is connectedto the one of the source and the drain of the transistor 101 and theinput terminal 102 to which the input voltage Vin is applied. The otherof the source and the drain of the transistor 124 is electricallyconnected to an output terminal of the diode 123 and one terminal of thecoil 122. The gate of the transistor 124 is electrically connected tothe third terminal of the internal control circuit 130.

Similarly to the transistor 101, an oxide semiconductor transistor whichis a power device is used as the transistor 124. When an oxidesemiconductor transistor is used as the transistor 124, damage of thetransistor 124 by application of the input voltage Vin which is highvoltage can be inhibited.

The output terminal of the diode 123 is electrically connected to theother of the source and the drain of the transistor 124 and the oneterminal of the coil 122. The power supply voltage VSS is applied to theinput terminal of the diode 123.

The one terminal of the coil 122 is electrically connected to the outputterminal of the diode 123 and the other of the source and the drain ofthe transistor 124. The other terminal of the coil 122 is electricallyconnected to one terminal of the capacitor 125 and the output terminal131 which outputs the output voltage Vout.

The one terminal of the capacitor 125 is electrically connected to theother terminal of the coil 122 and the output terminal 131 which outputsthe output voltage Vout. The power supply voltage VSS is applied to theother terminal of the capacitor 125.

Although the step-down voltage converter circuit 121 is described inFIG. 2, the voltage converter circuit 121 is not limited to thestep-down voltage converter circuit; instead of the step-down voltageconverter circuit, a step-up voltage converter circuit or a step-up/downvoltage converter circuit may be formed, if needed.

In FIG. 3, a DC-DC converter using a flyback voltage converter circuitwill be described.

The DC-DC converter in FIG. 3 includes the control circuit 111, avoltage converter circuit 141, the input terminal 102 to which the inputvoltage Vin is applied, the transistor 101, and the output terminal 131which outputs the output voltage Vout output from the voltage convertercircuit 141.

The voltage converter circuit 141 in FIG. 3 includes a transformer 149having a coil 142 and a coil 146, a transistor 144, a diode 143, and acapacitor 145.

One terminal of the coil 142 is electrically connected to the one of thesource and the drain of the transistor 101 and the input terminal 102 towhich the input voltage Vin is applied. The other terminal of the coil142 is electrically connected to one of a source and a drain of thetransistor 144.

The one of the source and the drain of the transistor 144 iselectrically connected to the other terminal of the coil 142. The powersupply voltage VSS is applied to the other of the source and the drainof the transistor 144. A gate of the transistor 144 is electricallyconnected to the third terminal of the internal control circuit 130.

One terminal of the coil 146 is electrically connected to an inputterminal of the diode 143. The power supply voltage VSS is applied tothe other terminal of the coil 146.

The input terminal of the diode 143 is electrically connected to the oneterminal of the coil 146. An output terminal of the diode 143 iselectrically connected to one terminal of the capacitor 145 and theoutput terminal 131.

The one terminal of the capacitor 145 is electrically connected to theoutput terminal of the diode 143 and the output terminal 131. The powersupply voltage VSS is applied to the other terminal of the capacitor145.

As described above, a DC-DC converter which is driven by the single highinput voltage Vin and which includes the voltage converter circuit 141and the control circuit 111 can be obtained.

In FIG. 4, a DC-DC converter using a forward voltage converter circuitwill be described.

The DC-DC converter in FIG. 4 includes the control circuit 111, avoltage converter circuit 151, the input terminal 102 to which the inputvoltage Vin is applied, the transistor 101, and the output terminal 131which outputs the output voltage Vout output from the voltage convertercircuit 151.

The voltage converter circuit 151 in FIG. 4 includes a transformer 159having a coil 152 and a coil 156, a transistor 154, a diode 153, a diode157, a coil 158, and a capacitor 155.

One terminal of the coil 152 is electrically connected to the one of thesource and the drain of the transistor 101 and the input terminal 102 towhich the input voltage Vin is applied. The other terminal of the coil152 is electrically connected to one of a source and a drain of thetransistor 154.

The one of the source and the drain of the transistor 154 iselectrically connected to the other terminal of the coil 152. The powersupply voltage VSS is applied to the other of the source and the drainof the transistor 154. A gate of the transistor 154 is electricallyconnected to the third terminal of the internal control circuit 130.

One terminal of the coil 156 is electrically connected to an inputterminal of the diode 153. The power supply voltage VSS is applied tothe other terminal of the coil 156.

The input terminal of the diode 153 is electrically connected to the oneterminal of the coil 156. An output terminal of the diode 153 iselectrically connected to an output terminal of the diode 157 and oneterminal of the coil 158.

The output terminal of the diode 157 is electrically connected to theoutput terminal of the diode 153 and the one terminal of the coil 158.The power supply voltage VSS is applied to an input terminal of thediode 157.

The one terminal of the coil 158 is electrically connected to the outputterminal of the diode 153 and the output terminal of the diode 157. Theother terminal of the coil 158 is electrically connected to one terminalof the capacitor 155 and the output terminal 131.

The one terminal of the capacitor 155 is electrically connected to theother terminal of the coil 158 and the output terminal 131. The powersupply voltage VSS is applied to the other terminal of the capacitor155.

As described above, a DC-DC converter which is driven by the single highinput voltage Vin and which includes the voltage converter circuit 151and the control circuit 111 can be obtained.

<Layered Structure and Manufacturing Process Thereof>

A layered structure in which the transistors included in the operationalamplifier 112 and the transistor 101 which is an oxide semiconductortransistor are stacked, and a manufacturing process of the layeredstructure will be described below. Note that in this embodiment,transistors including a silicon material in a channel formation regionare used as the transistors included in the operational amplifier 112.

First, as illustrated in FIG. 5A, an n-channel transistor 704 and ap-channel transistor 705 are formed over an insulating surface of asubstrate 700 by a known CMOS fabricating method. In this embodiment,the case where the n-channel transistor 704 and the p-channel transistor705 are formed with a single crystal semiconductor film which isseparated from a single crystal semiconductor substrate is given as anexample.

Specifically, an example of a manufacturing method of the single crystalsemiconductor film will be briefly described. First, an ion beamincluding accelerated ions is injected into the single crystalsemiconductor substrate. When the ion beam is injected, a crystalstructure in a region at a certain depth from the surface of thesemiconductor substrate is disordered. When the crystal structure isdisordered, a fragile layer which is locally weakened is formed. Thedepth at which the fragile layer is formed can be adjusted by theacceleration energy of the ion beam and the angle at which the ion beamenters. Then, the semiconductor substrate and the substrate 700 overwhich an insulating film 701 is formed are attached to each other sothat the insulating film 701 is provided therebetween. After thesemiconductor substrate and the substrate 700 are overlapped with eachother, a pressure of, approximately, greater than or equal to 1 N/cm²and less than or equal to 500 N/cm², preferably greater than or equal to11 N/cm² and less than or equal to 20 N/cm² is applied to part of thesemiconductor substrate and the substrate 700 to attach both thesubstrates. When the pressure is applied, bonding between thesemiconductor substrate and the insulating film 701 starts from theportion, which results in bonding of the entire surface where thesemiconductor substrate and the insulating film 701 are in close contactwith each other. Subsequently, heat treatment is performed, whereby verysmall voids that exist in the fragile layer are combined, so that thevery small voids increase in volume. As a result, the single crystalsemiconductor film which is part of the semiconductor substrate isseparated from the semiconductor substrate along the fragile layer. Theheat treatment is performed at a temperature not exceeding the strainpoint of the substrate 700. Then, the single crystal semiconductor filmis processed into a desired shape by etching or the like, so that anisland-shaped semiconductor film 702 and an island-shaped semiconductorfilm 703 can be formed.

The n-channel transistor 704 is formed using the island-shapedsemiconductor film 702 over the insulating film 701, and the p-channeltransistor 705 is formed using the island-shaped semiconductor film 703over the insulating film 701. The n-channel transistor 704 includes agate electrode 706, and the p-channel transistor 705 includes a gateelectrode 707. The n-channel transistor 704 includes an insulating film708 between the island-shaped semiconductor film 702 and the gateelectrode 706. The p-channel transistor 705 includes the insulating film708 between the island-shaped semiconductor film 703 and the gateelectrode 707.

Although there is no particular limitation on a substrate which can beused as the substrate 700, it is necessary that the substrate have atleast enough heat resistance to heat treatment performed later. Forexample, a glass substrate manufactured by a fusion method or a floatmethod, a quartz substrate, a ceramic substrate, or the like can be usedas the substrate 700. In the case where a glass substrate is used andthe temperature of heat treatment performed later is high, a glasssubstrate whose strain point is higher than or equal to 730° C. ispreferably used as the glass substrate. Further, a metal substrate suchas a stainless-steel substrate or a substrate in which an insulatingfilm is formed on the surface of a silicon substrate may be used aswell. Although a substrate formed of a flexible synthetic resin such asplastic generally has a lower resistance temperature than theaforementioned substrates, it may be used as long as being resistant toa processing temperature during manufacturing steps.

Note that although the case where the n-channel transistor 704 and thep-channel transistor 705 are formed using the single crystalsemiconductor film is described as an example in this embodiment, thepresent invention is not limited to this structure. For example, apolycrystalline semiconductor film which is formed over the insulatingfilm 701 by a vapor deposition method may be used. Alternatively, theabove semiconductor film may be crystallized with a known technique. Asthe known technique of crystallization, a laser crystallization methodusing a laser beam and a crystallization method using a catalyticelement are given. Alternatively, a crystallization method using acatalytic element and a laser crystallization method may be combined. Inthe case where a heat-resistant substrate such as a quartz substrate isused, a crystallization method combined with a thermal crystallizationmethod using an electrically heated oven, a lamp annealingcrystallization method using infrared light, a crystallization methodusing a catalytic element, or a high-temperature annealing method atapproximately 950° C., may be used. A transistor in which a channelformation region, a source region, and a drain region are formed in asingle crystal silicon substrate may also be used as each of then-channel transistor 704 and the p-channel transistor 705.

In FIG. 5A, after a conductive film is formed over the insulating film708, the conductive film is processed into a desired shape by etching orthe like, whereby a wiring 711 is formed together with the gateelectrode 706 and the gate electrode 707.

Next, as illustrated in FIG. 5A, an insulating film 712 is formed so asto cover the n-channel transistor 704, the p-channel transistor 705, andthe wiring 711. Note that although the case where the insulating film712 is formed in a single layer is described as an example in thisembodiment, the insulating film 712 is not necessarily a single layerand insulating films of two or more layers may be stacked as theinsulating film 712.

The insulating film 712 is formed using materials which can withstand atemperature of heat treatment in a later manufacturing step.Specifically, it is preferable to use silicon oxide, silicon nitride,silicon nitride oxide, silicon oxynitride, aluminum nitride, aluminumoxide, or the like for the insulating film 712.

The insulating film 712 may have its surface planarized by a CMP methodor the like.

Next, as illustrated in FIG. 5A, a gate electrode 713 of an oxidesemiconductor transistor 724 and a gate electrode 773 of an oxidesemiconductor transistor 781 are formed over the insulating film 712.The oxide semiconductor transistor 724 corresponds to the transistor 101in FIG. 1, FIG. 2, FIG. 3, and FIG. 4. The oxide semiconductortransistor 781 corresponds to the transistors 124 in FIG. 1 and FIG. 2,the transistor 144 in FIG. 3, and the transistor 154 in FIG. 4.

The gate electrode 713 and the gate electrode 773 can be formed with asingle-layer structure or a layered structure using a metal materialsuch as molybdenum, titanium, chromium, tantalum, tungsten, neodymium,or scandium; a conductive film using an alloy material that contains anyof these metal materials as a main component; or nitride of any of thesemetals. Note that aluminum or copper can also be used as such a metalmaterial if it can withstand the temperature of heat treatment to beperformed in a later process. Aluminum or copper is preferably combinedwith a refractory metal material so as to prevent a heat resistanceproblem and a corrosive problem. As the refractory metal material,molybdenum, titanium, chromium, tantalum, tungsten, neodymium, scandium,or the like can be used.

For example, as a two-layer structure of the gate electrode 713 and thegate electrode 773, the following structures are preferable: a two-layerstructure in which a molybdenum film is stacked over an aluminum film; atwo-layer structure in which a molybdenum film is stacked over a copperfilm; a two-layer structure in which a titanium nitride film or atantalum nitride film is stacked over a copper film; and a two-layerstructure in which a titanium nitride film and a molybdenum film arestacked. As a three-layer structure of the gate electrode 713 and thegate electrode 773, the following structure is preferable: a layeredstructure containing an aluminum film, an alloy film of aluminum andsilicon, an alloy film of aluminum and titanium, or an alloy film ofaluminum and neodymium in a middle layer and any of a tungsten film, atungsten nitride film, a titanium nitride film, and a titanium film in atop layer and a bottom layer.

Further, a light-transmitting oxide conductive film of indium oxide,indium oxide and tin oxide, indium oxide and zinc oxide, zinc oxide,zinc aluminum oxide, zinc aluminum oxynitride, zinc gallium oxide, orthe like can also be used as the gate electrode 713 and the gateelectrode 773.

The thickness of each of the gate electrode 713 and the gate electrode773 is in the range of 10 nm to 400 nm, preferably 100 nm to 200 nm. Inthis embodiment, after a conductive film for the gate electrode isformed to have a thickness of 150 nm by a sputtering method using atungsten target, the conductive film is processed (patterned) into adesired shape by etching, whereby the gate electrode 713 and the gateelectrode 773 are formed. Note that when end portions of the formed gateelectrodes are tapered, coverage with a gate insulating film stackedthereover is improved, which is preferable. Note that a resist mask maybe formed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

Next, as illustrated in FIG. 5B, a gate insulating film 714 is formedover the gate electrode 713 and the gate electrode 773. The gateinsulating film 714 can be formed to have a single-layer structure or alayered structure using one or more selected from a silicon oxide film,a silicon nitride film, a silicon oxynitride film, a silicon nitrideoxide film, an aluminum oxide film, an aluminum nitride film, analuminum oxynitride film, an aluminum nitride oxide film, a hafniumoxide film, and a tantalum oxide film by a plasma enhanced CVD method, asputtering method, or the like. It is preferable that the gateinsulating film 714 contain as little impurities such as moisture,hydrogen, or oxygen as possible. In the case of forming a silicon oxidefilm by a sputtering method, a silicon target or a quartz target is usedas a target, and oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

An oxide semiconductor that is made to be i-type (a highly purifiedoxide semiconductor) by removal of impurities is extremely sensitive tointerface state density and interface electric charge; thus, aninterface between the highly purified oxide semiconductor and the gateinsulating film 714 is important. Therefore, the gate insulating filmthat is in contact with the highly-purified oxide semiconductor needs tohave higher quality.

For example, a high-density plasma enhanced CVD using a microwave(frequency: 2.45 GHz) is preferably used, in which case an insulatingfilm which is dense, has high breakdown voltage, and is of high qualitycan be formed. This is because when the highly purified oxidesemiconductor is closely in contact with the high-quality gateinsulating film, the interface state can be reduced and interfaceproperties can be favorable.

Needless to say, a different film formation method such as a sputteringmethod or a plasma enhanced CVD method can be used as long as ahigh-quality insulating film can be formed as the gate insulating film714. In addition, any insulating film can be used as long as filmquality and characteristics of an interface with an oxide semiconductorare modified by heat treatment performed after deposition. In any case,any insulating film can be used as long as film quality as a gateinsulating film is high, interface state density between the gateinsulating film and an oxide semiconductor is decreased, and a favorableinterface can be formed.

The gate insulating film 714 may be formed to have a structure in whichan insulating film formed using a material having a high barrierproperty and an insulating film having lower proportion of nitrogen,such as a silicon oxide film or a silicon oxynitride film, are stacked.In this case, the insulating film such as a silicon oxide film or asilicon oxynitride film is formed between the insulating film having ahigh barrier property and the oxide semiconductor film. As theinsulating film having a high barrier property, a silicon nitride film,a silicon nitride oxide film, an aluminum nitride film, an aluminumnitride oxide film, or the like can be given, for example. Theinsulating film having a high barrier property is used, so thatimpurities in an atmosphere, such as moisture or hydrogen, or impuritiesin the substrate, such as an alkali metal or a heavy metal, can beprevented from entering the oxide semiconductor film, the gateinsulating film 714, or the interface between the oxide semiconductorfilm and another insulating film and the vicinity thereof. In addition,the insulating film having lower proportion of nitrogen such as asilicon oxide film or a silicon oxynitride film is formed so as to be incontact with the oxide semiconductor film, so that the insulating filmhaving a high barrier property can be prevented from being in contactwith the oxide semiconductor film directly.

For example, a silicon nitride film (SiN_(y) (y>0)) with a thickness ofgreater than or equal to 50 nm and less than or equal to 200 nm isformed by a sputtering method as a first gate insulating film, and asilicon oxide film (SiO_(x) (x>0)) with a thickness of greater than orequal to 5 nm and less than or equal to 300 nm is stacked over the firstgate insulating film as a second gate insulating film; thus, these filmsmay be used as the gate insulating film 714 having a thickness of 100nm. The thickness of the gate insulating film 714 may be set asappropriate depending on characteristics needed for the transistors andmay be approximately 350 nm to 400 nm.

In this embodiment, the gate insulating film 714 having a structure inwhich a silicon oxide film having a thickness of 100 nm formed by asputtering method is stacked over a silicon nitride film having athickness of 50 nm formed by a sputtering method is formed.

Note that the gate insulating film 714 is in contact with the oxidesemiconductor film to be formed later. Hydrogen contained in the oxidesemiconductor film adversely affects characteristics of the transistor;therefore, it is preferable that the gate insulating film 714 do notcontain hydrogen, a hydroxyl group, and moisture. In order that the gateinsulating film 714 contains as little hydrogen, a hydroxyl group, andmoisture as possible, it is preferable that an impurity adsorbed on thesubstrate 700, such as moisture or hydrogen, be eliminated and removedby preheating the substrate 700, over which the gate electrode 713 andthe gate electrode 773 are formed, in a preheating chamber of asputtering apparatus, as a pretreatment for film formation. Thetemperature for the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably, higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. Note that thispreheating treatment can be omitted.

Next, over the gate insulating film 714, an oxide semiconductor filmhaving a thickness of greater than or equal to 2 nm and less than orequal to 200 nm, preferably greater than or equal to 3 nm and less thanor equal to 50 nm, or more preferably greater than or equal to 3 nm andless than or equal to 20 nm is formed. Channel formation regions of theoxide semiconductor transistor 724 and the oxide semiconductortransistor 781 are formed in the oxide semiconductor film. The oxidesemiconductor film is formed by a sputtering method using an oxidesemiconductor target. Moreover, the oxide semiconductor film can beformed by a sputtering method under a rare gas (e.g., argon) atmosphere,an oxygen atmosphere, or a mixed atmosphere of a rare gas (e.g., argon)and oxygen.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust attached to a surface of the gate insulating film 714 ispreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of voltage to a target side, an RFpower source is used for application of voltage to a substrate side inan argon atmosphere to generate plasma in the vicinity of the substrateto modify a surface. Note that instead of an argon atmosphere, anitrogen atmosphere, a helium atmosphere, or the like may be used.Alternatively, an argon atmosphere to which oxygen, nitrous oxide, orthe like is added may be used. Alternatively, an argon atmosphere towhich chlorine, carbon tetrafluoride, or the like is added may be used.

As described above, as a material of the oxide semiconductor film, thefollowing oxide semiconductors can also be used: an In—Sn—Ga—Zn—O-basedoxide semiconductor which is a four-component metal oxide; anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, a Sn—Al—Zn—O-based oxide semiconductor which arethree-component metal oxides; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, an In—Mg—O-based oxide semiconductor, an In—Ga—O-basedoxide semiconductor which are two-component metal oxides; an In—O-basedoxide semiconductor, a Sn—O-based oxide semiconductor, and a Zn—O-basedoxide semiconductor which are one-component metal oxides.

In this embodiment, as the oxide semiconductor film, an In—Ga—Zn—O-basedoxide semiconductor thin film with a thickness of 30 nm, which isobtained by a sputtering method using a target including indium (In),gallium (Ga), and zinc (Zn), is used. As the target, a target having acomposition ratio in which In₂O₃:Ga₂O₃:ZnO=1:1:0.5 (molar ratio), atarget having a composition ratio in which In₂O₃:Ga₂O₃:ZnO=1:1:1 (molarratio), or a target having a composition ratio in whichIn₂O₃:Ga₂O₃:ZnO=1:1:2 (molar ratio) can be used. The filling rate of thetarget including In, Ga, and Zn is 90% or higher and 100% or lower, andpreferably 95% or higher and lower than 100%. With the use of the targetwith high filling rate, a dense oxide semiconductor film is formed.

When the purity of the target is set to 99.99% or higher, alkali metal,hydrogen atoms, hydrogen molecules, water, a hydroxyl group, hydride, orthe like mixed to the oxide semiconductor film can be reduced. Inaddition, when the target is used, the concentration of alkali metalsuch as lithium, sodium, or potassium can be reduced in the oxidesemiconductor film.

In this embodiment, the oxide semiconductor film is formed over thesubstrate 700 in such a manner that the substrate is held in thetreatment chamber kept at reduced pressure, a sputtering gas from whichhydrogen and moisture have been removed is introduced into the treatmentchamber while remaining moisture therein is removed, and the abovetarget is used. The substrate temperature may be higher than or equal to100° C. and lower than or equal to 600° C., preferably higher than orequal to 200° C. and lower than or equal to 400° C. in film formation.By forming the oxide semiconductor film in a state where the substrateis heated, the concentration of impurities included in the formed oxidesemiconductor film can be reduced. In addition, damage by sputtering canbe reduced. In order to remove remaining moisture in the treatmentchamber, an entrapment vacuum pump is preferably used. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. The evacuation unit may be a turbo pump provided with a cold trap.In the deposition chamber which is evacuated with the cryopump, forexample, a hydrogen atom, a compound containing a hydrogen atom, such aswater (H₂O), (more preferably, also a compound containing a carbonatom), and the like are removed, whereby the concentration of impuritiesincluded in the oxide semiconductor film formed in the depositionchamber can be reduced.

Moreover, when the leakage rate of the treatment chamber of thesputtering apparatus is set to lower than or equal to 1×10⁻¹⁰Pa·m³/second, entry of impurities such as an alkali metal or hydrideinto the oxide semiconductor film that is being formed by a sputteringmethod can be reduced. Further, with the use of an entrapment vacuumpump as an evacuation system, counter flow of impurities such as analkali metal, a hydrogen atom, a hydrogen molecule, water, a hydroxylgroup, or hydride from the evacuation system can be reduced.

As a sputtering as, a rare gas (typically argon), oxygen, or a mixed gasof a rare gas and oxygen is used as appropriate. It is preferable that ahigh-purity gas from which impurities such as hydrogen, water, ahydroxyl group, and hydride are removed be used as a sputtering gas.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulsed direct-current (DC) power supply is preferable becausedust generated in deposition can be reduced and the film thickness canbe made uniform.

In order that the oxide semiconductor film contains as little hydrogen,a hydroxyl group, and moisture as possible, it is preferable that animpurity adsorbed on the substrate 700, such as moisture or hydrogen, beeliminated and removed by preheating the substrate 700, over which filmsup to the gate insulating film 714 are formed, in a preheating chamberof a sputtering apparatus, as a pretreatment for film formation. Thetemperature for the preheating is higher than or equal to 100° C. andlower than or equal to 400° C., preferably, higher than or equal to 150°C. and lower than or equal to 300° C. As an exhaustion unit provided inthe preheating chamber, a cryopump is preferable. Note that thispreheating treatment can be omitted. This preheating may be similarlyperformed on the substrate 700 over which films up to and including anelectrode 716, an electrode 717, an electrode 718, an electrode 719, anelectrode 720, an electrode 779, and an electrode 780 are formed beforethe formation of an insulating film 723 which will be formed later.

Next, as illustrated in FIG. 5B, the oxide semiconductor film isprocessed (patterned) into a desired shape by etching or the like,whereby an island-shaped oxide semiconductor film 715 is formed over thegate insulating film 714 in a position where the island-shaped oxidesemiconductor film 715 overlaps with the gate electrode 713 and anisland-shaped oxide semiconductor film 775 is formed over the gateinsulating film 714 in a position where the island-shaped oxidesemiconductor film 775 overlaps with the gate electrode 773.

A resist mask for forming the island-shaped oxide semiconductor film 715and the island-shaped oxide semiconductor film 775 may be formed by anink-jet method. Formation of the resist mask by an inkjet method needsno photomask; thus, manufacturing cost can be reduced.

Note that etching for forming the island-shaped oxide semiconductor film715 and the island-shaped oxide semiconductor film 775 may be wetetching, dry etching, or both dry etching and wet etching. As an etchinggas used for dry etching, a gas containing chlorine (a chlorine-basedgas such as chlorine (Cl₂), boron trichloride (BCl₃), silicontetrachloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused. Alternatively, a gas containing fluorine (a fluorine-based gassuch as carbon tetrafluoride (CF₄), sulfur hexafluoride (SF₆), nitrogentrifluoride (NF₃), or trifluoromethane (CHF₃)), hydrogen bromide (HBr),oxygen (O₂), any of these gases to which a rare gas such as helium (He)or argon (Ar) is added, or the like can be used.

As the dry etching method, a parallel plate reactive ion etching (RIE)method or an inductively coupled plasma (ICP) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, ITO-07N (produced by KANTO CHEMICALCO., INC.) may be used. The etchant after the wet etching is removedtogether with the etched materials by cleaning. The waste liquidincluding the etchant and the material etched off may be purified andthe material may be reused. When a material such as indium included inthe oxide semiconductor films is collected from the waste liquid afterthe etching and reused, the resources can be efficiently used and thecost can be reduced.

Note that it is preferable that reverse sputtering be performed beforethe formation of a conductive film in a subsequent step so that a resistresidue or the like that is attached to surfaces of the oxidesemiconductor film 715, the oxide semiconductor film 775, and the gateinsulating film 714 is removed.

Note that, in some cases, the oxide semiconductor films formed bysputtering or the like include a large amount of moisture or hydrogen asimpurities. Moisture and hydrogen easily form a donor level and thusserve as impurities in the oxide semiconductor. Therefore, in oneembodiment of the present invention, heat treatment is performed on theoxide semiconductor film 715 and the oxide semiconductor film 775 in anatmosphere of nitrogen, oxygen, ultra-dry air, or a rare gas (argon,helium, or the like) in order to reduce impurities such as moisture orhydrogen in the oxide semiconductor films. It is desirable that thecontent of water in the gas be 20 ppm or less, preferably 1 ppm or less,and more preferably 10 ppb or less.

Heat treatment performed on the oxide semiconductor film 715 and theoxide semiconductor film 775 can eliminate moisture or hydrogen in theoxide semiconductor film 715 and the oxide semiconductor film 775.Specifically, heat treatment may be performed at higher than or equal to300° C. and lower than or equal to 700° C., preferably higher than orequal to 300° C. and lower than or equal to 500° C. For example, heattreatment may be performed at 500° C. for approximately greater than orequal to three minutes and less than or equal to six minutes. When anRTA method is used for the heat treatment, dehydration ordehydrogenation can be performed in a short time; therefore, treatmentcan be performed even at a temperature higher than the strain point of aglass substrate.

In this embodiment, an electrical furnace that is one of heat treatmentapparatuses is used.

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, a rapid thermal anneal (RTA)apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamprapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As the gas,an inert gas which does not react with an object to be processed by heattreatment, such as nitrogen or a rare gas such as argon is used.

Note that it is preferable that in the heat treatment, moisture,hydrogen, or the like be not contained in nitrogen or a rare gas such ashelium, neon, or argon. It is preferable that the purity of nitrogen orthe rare gas such as helium, neon, or argon which is introduced into aheat treatment apparatus be set to be 6N (99.9999%) or higher,preferably 7N (99.99999%) or higher (that is, the impurity concentrationis 1 ppm or lower, preferably 0.1 ppm or lower).

Through the above-described steps, the concentration of hydrogen in theoxide semiconductor film 715 and the oxide semiconductor film 775 can bereduced and the oxide semiconductor film 715 and the oxide semiconductorfilm 775 can be highly purified. Thus, the oxide semiconductor films canbe stabilized. In addition, heat treatment at a temperature of lowerthan or equal to the glass transition temperature makes it possible toform an oxide semiconductor film with a wide band gap in which carrierdensity is extremely low. Therefore, the transistor can be manufacturedusing a large-sized substrate, so that the productivity can beincreased. In addition, by using the oxide semiconductor film in whichthe hydrogen concentration is reduced and purity is improved, it ispossible to manufacture a transistor with high breakdown voltage and ahigh on-off ratio.

Note that in the case where the oxide semiconductor films are heated,although depending on materials or heating conditions of the oxidesemiconductor films, in some cases, the entire layer is not amorphousand crystals are formed at the surfaces of the oxide semiconductorfilms. The oxide semiconductor films preferably have non-single-crystalsin which crystals that are c-axis-oriented in a direction substantiallyperpendicular to the surfaces of the oxide semiconductor films.

Next, the insulating film 708, the insulating film 712, and the gateinsulating film 714 are partly etched, whereby contact holes reachingthe island-shaped semiconductor film 702, the island-shapedsemiconductor film 703, and the wiring 711 are formed.

Then, a conductive film is formed so as to cover the oxide semiconductorfilm 715 and the oxide semiconductor film 775 by a sputtering method ora vacuum evaporation method. After that, the conductive film isprocessed by etching or the like, so that the electrodes 716 to 720, theelectrode 779, and the electrode 780 each of which functions as a sourceelectrode, a drain electrode, or a wiring are formed as illustrated inFIG. 5C.

Note that the electrode 716 and the electrode 717 are in contact withthe island-shaped semiconductor film 702. The electrode 717 and theelectrode 718 are in contact with the island-shaped semiconductor film703. The electrode 719 is in contact with the wiring 711 and the oxidesemiconductor film 715. The electrode 720 is in contact with the oxidesemiconductor film 715. Note that the electrode 779 and the electrode780 are in contact with the oxide semiconductor film 775.

As a material of the conductive film for forming the electrodes 716 to720, the electrode 779, and the electrode 780, any of the followingmaterials can be used, for example: an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, or tungsten; an alloycontaining any of these elements; and an alloy film containing the aboveelements in combination. Alternatively, a structure may be employed inwhich a film of a refractory metal such as chromium, tantalum, titanium,molybdenum, or tungsten is stacked over or below a metal film ofaluminum or copper. Aluminum or copper is preferably combined with arefractory metal material so as to prevent a heat resistance problem anda corrosion problem. As the refractory metal material, molybdenum,titanium, chromium, tantalum, tungsten, neodymium, scandium, yttrium, orthe like can be used.

Further, the conductive film may have a single-layer structure or alayered structure of two or more layers. For example, a single-layerstructure of an aluminum film containing silicon; a two-layer structureof an aluminum film and a titanium film stacked thereover; a three-layerstructure in which a titanium film, an aluminum film, and a titaniumfilm are stacked in that order; and the like can be given.

The conductive film for forming the electrodes 716 to 720, the electrode779, and the electrode 780 may be formed using a conductive metal oxide.As a conductive metal oxide, indium oxide, tin oxide, zinc oxide, indiumoxide and tin oxide, indium oxide and zinc oxide, or the metal oxidematerial to which silicon or silicon oxide is added can be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor film 715 and the oxidesemiconductor film 775 are not removed in etching of the conductive filmas much as possible. Depending on the etching conditions, there are somecases in which an exposed portion of each of the island-shaped oxidesemiconductor film 715 and the island-shaped oxide semiconductor film775 is partly etched and thus a groove (a depression portion) is formed.

In this embodiment, a titanium film is used for the conductive film.Therefore, wet etching can be selectively performed on the conductivefilm using a solution (an ammonia hydrogen peroxide mixture) containingammonia and hydrogen peroxide water. Alternatively, dry etching may beperformed on the conductive film with the use of a gas containingchlorine (Cl₂), boron chloride (BCl₃), or the like.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted so as to have a plurality ofintensities. A resist mask formed with the use of a multi-tone mask hasa plurality of thicknesses and further can be changed in shape byetching; therefore, the resist mask can be used in a plurality ofetching steps for processing into different patterns. Therefore, aresist mask corresponding to at least two kinds or more of differentpatterns can be formed by one multi-tone mask. Thus, the number oflight-exposure masks can be reduced and the number of correspondingphotolithography steps can be also reduced, whereby simplification of aprocess can be realized.

Next, plasma treatment is performed using a gas such as N₂O, N₂, or Ar.By the plasma treatment, water or the like which attaches or is adsorbedto an exposed surfaces of the oxide semiconductor films is removed.Plasma treatment may be performed using a mixed gas of oxygen and argonas well.

After the plasma treatment, as illustrated in FIG. 6A, the insulatingfilm 723 is formed so as to cover the electrodes 716 to 720, theelectrode 779, the electrode 780, the oxide semiconductor film 715, andthe oxide semiconductor film 775. The insulating film 723 preferablycontains as little impurities such as moisture, hydrogen, and oxygen aspossible. An insulating film of a single layer or a plurality ofinsulating films stacked may be employed for the insulating film 723.When hydrogen is contained in the insulating film 723, entry of thehydrogen to the oxide semiconductor films or extraction of oxygen in theoxide semiconductor films by the hydrogen occurs, whereby a back channelportion of each of the oxide semiconductor films has lower resistance(n-type conductivity); thus, a parasitic channel might be formed.Therefore, it is important that a film formation method in whichhydrogen is not used be employed in order to form the insulating film723 containing as little hydrogen as possible. A material having a highbarrier property is preferably used for the insulating film 723. Forexample, as the insulating film having a high barrier property, asilicon nitride film, a silicon nitride oxide film, an aluminum nitridefilm, an aluminum nitride oxide film, or the like can be used.

When a plurality of insulating films stacked is used as the insulatingfilm 723, an insulating film having lower proportion of nitrogen, suchas a silicon oxide film or a silicon oxynitride film, is formed on theside closer to the oxide semiconductor film 715 and the oxidesemiconductor film 775 than the insulating film having a high barrierproperty. Then, the insulating film having a high barrier property isformed so as to overlap with the electrodes 716 to 720, the electrode779, the electrode 780, the oxide semiconductor film 715, and the oxidesemiconductor film 775 with the insulating film having lower proportionof nitrogen provided between the insulating film having a barrierproperty, and the electrodes 716 to 720, the electrode 779, theelectrode 780, the oxide semiconductor film 715, and the oxidesemiconductor film 775. By using the insulating film having a highbarrier property, the impurities such as moisture or hydrogen can beprevented from entering the oxide semiconductor film 715, the oxidesemiconductor film 775, the gate insulating film 714, the interfacebetween the oxide semiconductor film 715 and another insulating film andthe vicinity thereof, or the interface between the oxide semiconductorfilm 775 and another insulating film and the vicinity thereof. Inaddition, the insulating film having low proportion of nitrogen, such asa silicon oxynitride film formed in contact with the oxide semiconductorfilm 715 and the oxide semiconductor film 775 can prevent the insulatingfilm formed using a material having a high barrier property from beingin direct contact with the oxide semiconductor film 715 and the oxidesemiconductor film 775.

In this embodiment, the insulating film 723 having a structure in whicha silicon nitride film having a thickness of 100 nm formed by asputtering method is stacked over a silicon oxide film having athickness of 200 nm formed by a sputtering method is formed. Thesubstrate temperature in film formation may be higher than or equal toroom temperature and lower than or equal to 300° C. and in thisembodiment, is 100° C.

After the insulating film 723 is formed, heat treatment may beperformed. The heat treatment is performed under a nitrogen atmosphere,an ultra-dry air atmosphere, or a rare gas (e.g., argon and helium)atmosphere at preferably a temperature higher than or equal to 200° C.and lower than or equal to 400° C., for example, higher than or equal to250° C. and lower than or equal to 350° C. It is desirable that thecontent of water in the gas be 20 ppm or less, preferably 1 ppm or less,and more preferably 10 ppb or less. In this embodiment, for example,heat treatment is performed at 250° C. in a nitrogen atmosphere for 1hour. Alternatively, RTA treatment for a short time at a hightemperature may be performed before the formation of the electrodes 716to 720, the electrode 779, and the electrode 780 in a manner similar tothat of the previous heat treatment performed on the oxide semiconductorfilms for reduction of moisture or hydrogen. Even when oxygen defectsare generated in the oxide semiconductor film 715 and the oxidesemiconductor film 775 because of the previous heat treatment performedon the oxide semiconductor films, by performing heat treatment after theinsulating film 723 containing oxygen is provided, oxygen is supplied tothe oxide semiconductor film 715 and the oxide semiconductor film 775from the insulating film 723. A structure can be provided in which, bysupplying oxygen to the oxide semiconductor film 715 and the oxidesemiconductor film 775, oxygen defects serving as donors are reduced inthe oxide semiconductor film 715 and the oxide semiconductor film 775 sothat oxygen is contained to satisfy the stoichiometric proportion or tobe greater than or equal to the stoichiometric proportion. It ispreferable that the proportion of oxygen in the oxide semiconductor film715 and the oxide semiconductor film 775 be higher than that in thestoichiometric proportion. As a result, the oxide semiconductor film 715and the oxide semiconductor film 775 can be made to be substantiallyi-type and variation in electrical characteristics of the transistorsdue to oxygen defects can be reduced; thus, the electricalcharacteristics can be improved. The timing of this heat treatment isnot particularly limited as long as it is after the formation of theinsulating film 723. When this heat treatment also serves as heattreatment in another step (e.g., heat treatment at the time of formationof a resin film or heat treatment for reducing the resistance of atransparent conductive film), the oxide semiconductor film 715 and theoxide semiconductor film 775 can be or substantially i-type without anincrease in the number of steps.

Moreover, the oxygen defects that serve as donors in the oxidesemiconductor film 715 and the oxide semiconductor film 775 may bereduced by subjecting the oxide semiconductor film 715 and the oxidesemiconductor film 775 to heat treatment under an oxygen atmosphere sothat oxygen is added to the oxide semiconductor. The heat treatment isperformed at a temperature, for example, higher than or equal to 100° C.and lower than 350° C., preferably higher than or equal to 150° C. andlower than 250° C. It is preferable that an oxygen gas used for the heattreatment under an oxygen atmosphere do not include water, hydrogen, orthe like. Alternatively, the purity of the oxygen gas which isintroduced into the heat treatment apparatus is preferably greater thanor equal to 6N (99.9999%) or more preferably greater than or equal to 7N(99.99999%) (that is, the impurity concentration in the oxygen is lessthan or equal to 1 ppm, or preferably less than or equal to 0.1 ppm).

Alternatively, an ion implantation method, an ion doping method, or thelike may be employed to add oxygen to the oxide semiconductor film 715and the oxide semiconductor film 775 so that oxygen defects as donorsare reduced. For example, oxygen made to be plasma with a microwave of2.45 GHz may be added to the oxide semiconductor film 715 and the oxidesemiconductor film 775.

Through the above steps, the oxide semiconductor transistor 724 and theoxide semiconductor transistor 781 are formed.

The oxide semiconductor transistor 724 includes the gate electrode 713,the gate insulating film 714 over the gate electrode 713, the oxidesemiconductor film 715 which is over the gate insulating film 714 andoverlaps with the gate electrode 713, a pair of the electrode 719 andthe electrode 720 formed over the oxide semiconductor film 715, and theinsulating film 723 which is formed over the oxide semiconductor film715.

Similarly, the oxide semiconductor transistor 781 includes the gateelectrode 773, the gate insulating film 714 over the gate electrode 773,the oxide semiconductor film 775 which is over the gate insulating film714 and overlaps with the gate electrode 773, a pair of the electrode779 and the electrode 780 formed over the oxide semiconductor film 775,and the insulating film 723 which is formed over the oxide semiconductorfilm 775.

Note that the oxide semiconductor transistor 724 in FIG. 6A has achannel-etched structure in which part of the oxide semiconductor film715 is etched between the electrode 719 and the electrode 720.Similarly, the oxide semiconductor transistor 781 in FIG. 6A has achannel-etched structure in which part of the oxide semiconductor film775 is etched between the electrode 779 and the electrode 780.

Although description is given using single-gate transistors as the oxidesemiconductor transistor 724 and the oxide semiconductor transistor 781,a multi-gate transistor including a plurality of channel formationregions by including a plurality of the gate electrodes that areelectrically connected to each other may be formed as needed.

The n-channel transistor 704 and the p-channel transistor 705illustrated in FIG. 6A can be used as the transistors included in theoperational amplifiers 112 illustrated in FIG. 2, FIG. 3, and FIG. 4.The oxide semiconductor transistor 724 illustrated in FIG. 6A can beused as the transistors 101 illustrated in FIG. 1, FIG. 2, FIG. 3, andFIG. 4. The oxide semiconductor transistor 781 illustrated in FIG. 6Acan be used as the transistors 124 in FIG. 1 and FIG. 2, the transistor144 in FIG. 3, and the transistor 154 in FIG. 4.

Accordingly, the transistors included in the operational amplifier 112,the transistor 101 which is an oxide semiconductor transistor, and thetransistor of the voltage converter circuit (the transistor 124, thetransistor 144, or the transistor 154) can be stacked over the substrate700 with the insulating film 712 interposed between the transistorsincluded in the operational amplifier 112 and the transistor 101 and thetransistor of the voltage converter circuit. Thus, the increase of theoccupied area of the DC-DC converter can be suppressed.

In addition, the transistor 101 which is an oxide semiconductortransistor and the transistor of the voltage converter circuit (thetransistor 124, the transistor 144, or the transistor 154) can be formedin the same manufacturing process; therefore, the number ofmanufacturing steps and a manufacture cost can be reduced.

A layered structure which is different from that in FIG. 6A will beillustrated in FIG. 6B.

FIG. 6B illustrates the n-channel transistor 704 and the p-channeltransistor 705. Further, in FIG. 6B, a bottom-gate oxide semiconductortransistor 725 and a bottom-gate oxide semiconductor transistor 751which have channel protective structures using an oxide semiconductorfilm are formed over the n-channel transistor 704 and the p-channeltransistor 705.

The oxide semiconductor transistor 725 includes a gate electrode 730which is formed over the insulating film 712, a gate insulating film 731which is over the gate electrode 730, an island-shaped oxidesemiconductor film 732 which overlaps with the gate electrode 730 andwhich is over the gate insulating film 731, a channel protective film733 which overlaps with the gate electrode 730 and which is over theisland-shaped oxide semiconductor film 732, an electrode 734 and anelectrode 735 which are formed over the oxide semiconductor film 732,and an insulating film 736 which is formed over the electrode 734, theelectrode 735, and the channel protective film 733.

The oxide semiconductor transistor 751 includes a gate electrode 750which is formed over the insulating film 712, the gate insulating film731 which is over the gate electrode 750, an island-shaped oxidesemiconductor film 752 which overlaps with the gate electrode 750 andwhich is over the gate insulating film 731, a channel protective film753 which overlaps with the gate electrode 750 and which is over theisland-shaped oxide semiconductor film 752, an electrode 754 and anelectrode 755 which are formed over the oxide semiconductor film 752,and the insulating film 736 which is formed over the electrode 754, theelectrode 755, and the channel protective film 753.

The channel protective film 733 and the channel protective film 753 canprevent a portion of the oxide semiconductor film 732 and the oxidesemiconductor film 752, which each serves as a channel formation region,from being damaged in a later step, for example, reduction in thicknessdue to plasma or an etchant in etching. Therefore, reliability of thetransistors can be improved.

An inorganic material containing oxygen (silicon oxide, silicon nitrideoxide, silicon oxynitride, aluminum oxide, aluminum oxynitride, or thelike) can be used for the channel protective film 733 and the channelprotective film 753. The channel protective film 733 and the channelprotective film 753 can be formed by a vapor deposition method such as aplasma enhanced CVD method or a thermal CVD method, or a sputteringmethod. After the formation of the channel protective film 733 and thechannel protective film 753, the shapes thereof are processed byetching. Here, the channel protective film 733 and the channelprotective film 753 are formed in such a manner that a silicon oxidefilm is formed by a sputtering method and processed by etching using amask formed by photolithography.

An inorganic material containing oxygen is used for the channelprotective film 733 and the channel protective film 753, whereby astructure can be provided in which oxygen is supplied from the channelprotective film 733 to the oxide semiconductor film 732 and oxygen issupplied from the channel protective film 753 to the oxide semiconductorfilm 752 and oxygen defects serving as donors are reduced so that oxygenis contained to satisfy the stoichiometric proportion or to be greaterthan or equal to the stoichiometric proportion even when the oxygendefects are generated in the oxide semiconductor film 732 and the oxidesemiconductor film 752 by heat treatment performed to reduce moisture orhydrogen. The oxide semiconductor film 732 and the oxide semiconductorfilm 752 preferably contain oxygen whose composition exceeds thestoichiometric composition. Thus, the channel formation region can bemade to be close to i-type. The channel formation region can be made tobe close to i-type, so that a variation in electrical characteristics ofthe oxide semiconductor transistor 725 and the oxide semiconductortransistor 751 due to oxygen defects can be reduced; accordingly, theelectrical characteristics can be improved.

The n-channel transistor 704 and the p-channel transistor 705illustrated in FIG. 6B can be used as the transistors included in theoperational amplifiers 112 illustrated in FIG. 2, FIG. 3, and FIG. 4.The oxide semiconductor transistor 725 illustrated in FIG. 6B can beused as the transistors 101 illustrated in FIG. 1, FIG. 2, FIG. 3, andFIG. 4. The oxide semiconductor transistor 751 illustrated in FIG. 6Bcan be used as the transistors 124 in FIG. 1 and FIG. 2, the transistor144 in FIG. 3, and the transistor 154 in FIG. 4.

Accordingly, the transistors included in the operational amplifier 112,the transistor 101 which is an oxide semiconductor transistor, and thetransistor of the voltage converter circuit (the transistor 124, thetransistor 144, or the transistor 154) can be stacked over the substrate700 with the insulating film 712 interposed between the transistorsincluded in the operational amplifier 112 and the transistor 101 and thetransistor of the voltage converter circuit. Thus, the increase of theoccupied area of the DC-DC converter can be suppressed.

In addition, the transistor 101 which is an oxide semiconductortransistor and the transistor of the voltage converter circuit (thetransistor 124, the transistor 144, or the transistor 154) can be formedin the same manufacturing process; therefore, the number ofmanufacturing steps and a manufacture cost can be reduced.

FIG. 6C illustrates the n-channel transistor 704 and the p-channeltransistor 705. In addition, a bottom-contact oxide semiconductortransistor 726 and a bottom-contact oxide semiconductor transistor 760including an oxide semiconductor film are formed over the n-channeltransistor 704 and the p-channel transistor 705 in FIG. 6C.

The oxide semiconductor transistor 726 includes a gate electrode 741which is formed over the insulating film 712, a gate insulating film 742which is over the gate electrode 741, an electrode 743 and an electrode744 which are over the gate insulating film 742, an oxide semiconductorfilm 745 which overlaps with the gate electrode 741 with the gateinsulating film 742 therebetween, and an insulating film 746 which isformed over the oxide semiconductor film 745.

The oxide semiconductor transistor 760 includes a gate electrode 761which is formed over the insulating film 712, the gate insulating film742 which is over the gate electrode 761, an electrode 763 and anelectrode 764 which are over the gate insulating film 742, an oxidesemiconductor film 765 which overlaps with the gate electrode 761 withthe gate insulating film 742 therebetween, and the insulating film 746which is formed over the oxide semiconductor film 765.

The n-channel transistor 704 and the p-channel transistor 705illustrated in FIG. 6C can be used as the transistors included in theoperational amplifiers 112 illustrated in FIG. 2, FIG. 3, and FIG. 4.The oxide semiconductor transistor 726 illustrated in FIG. 6C can beused as the transistors 101 illustrated in FIG. 1, FIG. 2, FIG. 3, andFIG. 4. The oxide semiconductor transistor 760 illustrated in FIG. 6Ccan be used as the transistors 124 in FIG. 1 and FIG. 2, the transistor144 in FIG. 3, and the transistor 154 in FIG. 4.

Accordingly, the transistors included in the operational amplifier 112,the transistor 101 which is an oxide semiconductor transistor, and thetransistor of the voltage converter circuit (the transistor 124, thetransistor 144, or the transistor 154) can be stacked over the substrate700 with the insulating film 712 interposed between the transistorsincluded in the operational amplifier 112 and the transistor 101 and thetransistor of the voltage converter circuit. Thus, the increase of theoccupied area of the DC-DC converter can be suppressed.

In addition, the transistor 101 which is an oxide semiconductortransistor and the transistor of the voltage converter circuit (thetransistor 124, the transistor 144, or the transistor 154) can be formedin the same manufacturing process; therefore, the number ofmanufacturing steps and a manufacture cost can be reduced.

This application is based on Japanese Patent Application serial no.2010-270316 filed with Japan Patent Office on Dec. 3, 2010, the entirecontents of which are hereby incorporated by reference.

1. A DC-DC converter comprising: an input terminal to which inputvoltage is applied; a voltage converter circuit that is connected to theinput terminal and includes a first transistor; a control circuit thatis configured to control the voltage converter circuit and includes asecond transistor including a silicon material in a channel formationregion; and a third transistor that is provided between the inputterminal and the control circuit and configured to convert the inputvoltage into power supply voltage that is lower than the input voltage,wherein the first transistor and the third transistor are transistorsincluding an oxide semiconductor material in channel formation regions,and wherein the first transistor and the third transistor are stackedover the second transistor with an insulating film provided between thefirst and third transistors and the second transistor.
 2. The DC-DCconverter according to claim 1, wherein the oxide semiconductor materialis any one of an In—Sn—Ga—Zn—O-based oxide semiconductor, anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, a Sn—Al—Zn—O-based oxide semiconductor, an In—Zn—O-basedoxide semiconductor, a Sn—Zn—O-based oxide semiconductor, anAl—Zn—O-based oxide semiconductor, a Zn—Mg—O-based oxide semiconductor,a Sn—Mg—O-based oxide semiconductor, an In—Mg—O-based oxidesemiconductor, an In—Ga—O-based oxide semiconductor, an In—O-based oxidesemiconductor, a Sn—O-based oxide semiconductor, and a Zn—O-based oxidesemiconductor.
 3. The DC-DC converter according to claim 1, wherein thevoltage converter circuit is a step-down voltage converter circuit. 4.The DC-DC converter according to claim 1, wherein the voltage convertercircuit is a flyback voltage converter circuit.
 5. The DC-DC converteraccording to claim 1, wherein the voltage converter circuit is a forwardvoltage converter circuit.
 6. A method for manufacturing a DC-DCconverter, comprising the steps of: forming a first transistor using asilicon material in a first channel formation region, over an insulatingsurface; forming an insulating film covering the first transistor; andforming a second transistor including an oxide semiconductor material ina second channel formation region and a third transistor using the oxidesemiconductor material in a third channel formation region over theinsulating film, wherein a voltage converter circuit includes the secondtransistor, wherein a control circuit configured to control the voltageconverter circuit includes the first transistor, and wherein the thirdtransistor is provided between an input terminal and the controlcircuit, and converts input voltage applied to the input terminal intopower supply voltage that is lower than the input voltage.
 7. The methodfor manufacturing a DC-DC converter, according to claim 6, wherein theoxide semiconductor material is any one of an In—Sn—Ga—Zn—O-based oxidesemiconductor, an In—Ga—Zn—O-based oxide semiconductor, anIn—Sn—Zn—O-based oxide semiconductor, an In—Al—Zn—O-based oxidesemiconductor, a Sn—Ga—Zn—O-based oxide semiconductor, anAl—Ga—Zn—O-based oxide semiconductor, a Sn—Al—Zn—O-based oxidesemiconductor, an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-basedoxide semiconductor, an Al—Zn—O-based oxide semiconductor, aZn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxide semiconductor,an In—Mg—O-based oxide semiconductor, an In—Ga—O-based oxidesemiconductor, an In—O-based oxide semiconductor, a Sn—O-based oxidesemiconductor, and a Zn—O-based oxide semiconductor.
 8. The method formanufacturing a DC-DC converter according to claim 6, wherein thevoltage converter circuit is a step-down voltage converter circuit. 9.The method for manufacturing a DC-DC converter according to claim 6,wherein the voltage converter circuit is a flyback voltage convertercircuit.
 10. The method for manufacturing a DC-DC converter according toclaim 6, wherein the voltage converter circuit is a forward voltageconverter circuit.